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Re: Toroid ballast design



Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Steve,

I think your suggestion is a good one. The variac had a tapered winding with a total number of turns of 200. It was a 120Vac input and 0-120Vac output, so the volts per turn with its 3 sq inch core was 0.6. With twice the core area, it seems like 1.2 volts per turn would be appropriate. With 200 turns, I could go to 240 Vac and keep things the same (from the core point of view).

I think your comment about two stacked cores needing half the number of turns is correct if the applied voltage stays the same. But I want to use it in a 240V (actually 280V) application so I really need more than 200 turns I believe.

Gerry R.

Original poster: Steve Conner <steve@xxxxxxxxxxxx>


I dont know where these cores saturate and if 24600 gauss is no problem, the fewer number of turns will mean less winding loss.

I've heard that 17000 gauss is about where transformer iron starts to saturate. Commercial transformer designs seem to aim for about this 17kG peak figure. 24600 seems a bit "Out there" :-o


One handy rule of thumb I heard when designing gapped ballasts is that gapping the core doesn't change the volt-second capability. (To a first order at least.) So the number of turns you need to avoid saturation is the same as for an ungapped transformer. Hence a good starting point with a gapped variac core would be "The same number of turns as the variac had".

I guess with two stacked cores, you only need half the number of turns, or is it 1/sqrt(2)?

Steve Conner
http://www.scopeboy.com/